This tends not to help much because bras from different manufacturers vary so much. Even bras from the same manufacturer in different styles vary in size. The only way I can make sure I'm buying a bra that fits is to try it on:-(

If you think you might be a smaller back/larger cup size you could try Bravissimo. I thought I was a bigger back/smaller cup until I went to Rigby and Peller for my wedding undergarments and they sold me a bra from a different manufacturer, not their own brand and that size has fit me ever since, but Bravissimo would have done the same. They also have a sports bra that even keeps ME from flopping about painfully. M&S hadn't got my size right before, ever.